DESCRIPTION (from applicant's abstract): The third variable loop (V3) of the envelope glycoprotein, gpl20, is a major neutralizing determinant of HIV-1. Segments of V3 form the binding site for the gpl20 co-receptors on T-cells and macrophages and its sequence determines the virus phenotype, i.e. whether it binds the CCR5 chemokine receptor and infects macrophages (M-tropic virus), or it binds CXCR4 and infects T-cells (T-tropic virus). Antibody binding to V3, or its deletion, prevent the viral fusion with the target cells, thus abolishing its infectivity. Neutralization of a variety of HIV-1 strains, including field isolates, has been shown with serum obtained after immunization with antigens containing V3 peptides of different strains. Immunization with a "cocktail" of peptides may overcome the V3 sequence variability problem. Phase I/II clinical trials of candidate vaccines containing V3 peptides have been conducted. Passive immunization with a chimeric form of the murine strain-specific anti-gpl20 antibody 0.5beta, directed against V3, was found to protect chimpanzees from HIV-1 infection. The human monoclonal antibody 447-52D was found to neutralize a broad spectrum of laboratory-adapted HIV-1 strains and both syncytium inducing (SI) and non-syncytium inducing (NSI) field isolates. The structure of V3 peptides of different HIV-1 strains, including a primary-isolate V3, bound to 447-52D will be determined using uniformly 13C, 15N and 2H labeled peptides in combination with TROSY and isotope-edited NMR experiments. The Fab-peptide interactions will be studied as well. In addition, the solution structure of the whole cyclic V3MN bound to 0.5beta and V3JR-FL bound to 447-52D will be determined. The structural differences between V3 of T-tropic and M-tropic viruses will be examined. The importance of each V3 residue for 0.5beta binding and the conformational changes in 0.5beta upon V3 binding will be studied. Our studies will provide a structural insight into the different determinants within V3: the regions involved in co-receptor binding, the residues that select the co-receptor used by the virus and the variable spots that help the virus evade the immune system. Our studies may help to determine the optimal length of peptide immunogens to be used in a peptide-based vaccine against HIV, to design constrained peptides immunogens for better cross-reactivity of anti-peptide antibodies with the virus, and to select an appropriate mixture of V3 peptides from different HIV isolates to induce a broadly neutralizing antibody response.